Diagnosis method for an antenna connection

ABSTRACT

A method for testing a reliable connection of an antenna includes a coil (L) or something similar connected to a reference potential on one side and to the output of an amplifier on the other side, a first capacitor (C 2 ) arranged in a parallel position to the coil ( 1 ) and a second capacitor (C 3 ) arranged between the terminal of the coil ( 1 ) and the reference potential. The method consists in emitting a signal by the amplifier, carrying out a first voltage measurement on the antenna terminal during a transient state, and in carrying out the second voltage measurement on the same antenna terminal in a steady-state condition. The method can be used for vehicle LF antennas.

This application is a national stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/EP03/09454, filed Aug. 27, 2003, whichclaims priority from French Application No. 0212084, filed Sept. 30,2002, the specification of which is incorporated herein by reference inits entirety.

The present invention relates to a diagnostic procedure relating to theconnection of an antenna, in particular an antenna used for transmittinglow frequency signals in a motor vehicle.

For a motor vehicle, the use of a low frequency (LF) antenna for dialogwith an external badge or even for monitoring tire pressure is known.

In the first case, it is assumed that the vehicle is equipped with ahands-free system for accessing the vehicle and, where appropriate, forstarting it without having to use a mechanical key. The user of thevehicle is then simply provided with a badge which is detected andrecognized by a control and management device linked to antennaspositioned on board the vehicle. If the badge is identified by thecontrol and management device as being a badge authorized for thevehicle, the wearer of this badge can enter into the vehicle simply bygrasping a door handle and, where necessary, start the engine of thevehicle simply by pressing a button.

For such a hands-free system, several LF antennas are provided. Eachantenna is driven by a driver device, the latter, where necessary,driving a number of antennas. Normally, there are four drivers in avehicle equipped with a hands-free access system (and, whereappropriate, starting system): one driver for the antennas locatedoutside the vehicle on its left-hand side, one for the external antennaslocated on the right, one for the external antennas located at the rearof the vehicle and a final one for the antennas located inside thevehicle.

In some vehicles equipped with a tire pressure monitoring system, an LFantenna is located in the vicinity of each wheel of the vehicle. Theseantennas are then each driven by a driver, which is in turn linked to acomputer placed on board the vehicle.

One problem that arises when installing a hands-free system or a tirepressure monitoring system is how to check that each antenna is indeedlinked to its driver. The latter is normally incorporated in a controland management computer inside the passenger compartment of the vehicle.

Document DE-198 20 207 describes a diagnostic device for an antenna. Thediagnosis is provided by measuring an inductance. A signal is sent tothe antenna and the phase difference between the excitation signal andthe resonance voltage induced by this excitation is measured. Theprocedure then applied to check the correct connection of an antenna isrelatively lengthy and complicated. Also, the results are not the sameif one or more antennas are connected to one and the same driver.Furthermore, special equipment is needed to apply this procedure.

The object of the present invention is therefore to provide a procedurethat is simple to implement for detecting the presence and satisfactoryconnection of an antenna to its control device. This procedurepreferably detects whether the antenna being checked is presenting ashort circuit to ground.

To this end, it proposes a diagnostic procedure relating to theconnection of an antenna including a coil or similar linked on the onehand to a reference potential and on the other hand to an output of anamplifier, a first capacitor being mounted in parallel with the coil anda second capacitor being inserted between a terminal of the coil and thereference potential.

According to the invention, this procedure comprises the followingsteps:

-   a) transmission of a signal by the amplifier,-   b) first measurement of the voltage at a terminal of the antenna    during the transient state provoked by the transmission of the    signal, and-   c) second measurement of the voltage at the same terminal of the    antenna in the steady state.

Although normally the measurements are performed in steady-stateoperation, the procedure proposed here entails taking a measurement inthe transient state. This first measurement gives indications concerningthe installation of the antenna and its connection. If only the secondmeasurement is carried out, it is not possible to detect whether theantenna is connected or not. Of course, the voltage measured in thesteady state is low in the first case whereas, when the antenna isdisconnected, the measured voltage is relatively high, because the firstcapacitor is then charged. However, if the antenna is connected butpresents a short circuit to ground, the second measurement, when carriedout on its own, will report as its result that the connection is set upand correct. The first measurement during the transient state is thenused to distinguish the case where the antenna is correctly connectedfrom the case where it presents a short circuit to ground. In the firstcase, the first measurement gives a high voltage whereas, in the secondcase, the voltage measured on this first measurement is close to zero.

A diagnostic procedure according to the invention can, for example, becarried out on powering up the antenna.

Preferably, the signal sent by the amplifier to carry out the diagnosticis not modulated.

The signal at the amplifier output is, for example, a signal of the typepresenting a rise time followed by a pulse duration and finally a falltime. In this case, the first measurement is preferably carried outduring the rise time, for example in the second half of the signal rise.This first measurement can also be carried out right at the start of thepulse duration. The second measurement is, for example, carried outduring the pulse duration.

The procedure according to the invention can also be adapted todiagnosing a number of antennas. Each antenna is then, for example,connected to an input stage of a multiplexer and a resistor positionedbetween the antenna and the input stage of the multiplexer limits thecurrent in the corresponding multiplexer input stage.

Other details and advantages of the present invention will becomeapparent from the description that follows, given with reference to theappended diagrammatic drawings, in which:

FIG. 1 is a schematic diagram of a circuit for implementing a procedureaccording to the invention,

FIG. 2 is an electrical circuit diagram corresponding to the circuit ofFIG. 1 associated with a read circuit,

FIG. 3 is a diagram representing the shape of the signal sent to theantenna to check its connection,

FIG. 4 is a diagram showing the shape of the different signals when theantenna is correctly connected,

FIG. 5 corresponds to FIG. 4, with a different time scale, when theantenna is not connected, and

FIG. 6 corresponds to FIG. 5 for an antenna connected but presenting ashort circuit to ground.

The description provided below refers to an antenna installed on a motorvehicle and intended for use in a hands-free system, enabling a wearerof a badge to access the interior of the vehicle and, where appropriate,to start it without using a mechanical key. Such a system is known tothose skilled in the art and is not described in detail here.

The description below considers a low frequency (LF) antenna designed tosend signals at a frequency of 125 kHz. Naturally, the invention canalso be applied to an antenna sending signals of different frequenciesand for other applications.

Conventionally, a sinusoidal signal generator (in this case with afrequency of 125 kHz) supplies a carrier to a modulator. The latterreceives from a management and control device the data that has to betransmitted by the antenna and “assembles” the signal from the generatorwith the data that it receives to supply the signal to be sent. Thissignal then passes into an amplifier device, commonly called a “driver”.All these various elements are normally arranged inside the same box, orcomputer, placed, for example, on the dashboard of a vehicle. Only theantenna is positioned remotely from the box. This antenna is, forexample, incorporated in a door handle or the trunk of the vehicle orelse positioned inside the vehicle passenger compartment. The antenna islinked to the box, or computer, via a cable and connectors. Normally, anumber of antennas are connected to one and the same box.

Numerous connections therefore need to be provided. The satisfactoryoperation of the hands-free system then depends on the quality of theseconnections. It is therefore important to check when installing that theconnections are made correctly. FIGS. 1 and 2 show a diagnostic circuitfor simply checking the presence (or detecting the absence) of anantenna while checking that the latter does not present a short circuitto ground or to positive.

FIG. 1 diagrammatically represents a coil 2 corresponding to the antennadiscussed previously and an output amplifier 4 of the driver controllingthis antenna. Three capacitors C1, C2 and C3 and a resistor R1 can alsobe seen in FIG. 1. These capacitors and this resistor, like the outputamplifier 4, are located with the computer, for example inside thevehicle passenger compartment, while the coil 2 is located outside thiscomputer.

The amplifier 4 is powered at a voltage Valim that is assumed, forexample, to be equal to 24 V. The capacitor C1 is located at the outputof this amplifier 4. Such a capacitor is almost always present at theoutput of an amplifier in the case of a transmitter. It is used toeliminate the DC component of the amplifier output signal in order tosend a purely AC signal to the sending coil 2.

The capacitor C2 is mounted in parallel with the coil 2 whereas thecapacitor C3 is mounted in series between the reference potential andthe resonant circuit assembly formed by the capacitor C2 and the coil 2.

Thus, at the output of the amplifier 4, the three capacitors C1, C2 andC3 are mounted in series whereas the coil 2 is mounted in parallel withthe capacitor C2.

The capacitor C2 reduces the current at the output of the amplifier 4.The signals sent to the antenna have a frequency of 125 kHz and theresonant circuit formed by the coil 2 of inductance L and the capacitorC2 presents a high impedance at this frequency.

The capacitor C3 protects the antenna from any short circuit that mightoccur on the positive terminal of the battery supplying the transmitter.

By way of nonlimiting example, a few numeric values are given below:C1=C3=1 μF,C2=37 nF,L=38 μHR1=2.2 kΩ.

FIG. 2 contains the same elements as those represented in FIG. 1 anddescribed above. An input stage of a multiplexer is also represented onthe right of this figure. This device is intended to measure the voltagevdiag read directly at the terminal of the coil 2 located alongside theamplifier 4, in other words the voltage (analog signal) prevailingbetween the capacitor C1 and the coil 2. Conventionally, to limit theinput current in the multiplexer, a resistor R2 is provided. Such amultiplexer input stage is known to those skilled in the art. It mainlyincludes a power supply to the read circuit represented by a generatorof a voltage V3. Diodes are provided to protect the read logic circuit.The input impedance of this circuit is symbolized by a resistor R3. Thevoltage given at the output by this logic circuit is called Vmux. Itreflects the voltage Vdiag mentioned previously. The use of amultiplexer limits the number of input/outputs of the microprocessorused for the measurement.

By way of nonlimiting numerical example, there is, for example, a powersupply voltage Vdd of 5 V with the following resistors:R2=47 kΩ and R3=100 MΩ.

FIG. 3 shows an example of a signal that can power the coil 2 to providea connection diagnostic. This signal first presents an initial value,for example zero. This period where the output voltage takes the initialvalue is followed by a rise time to reach a maximum value. Apredetermined pulse duration is followed by a fall time to return to theinitial value. This signal is, for example, a periodic signal of periodP. By way of nonlimiting numerical example, the following values can beapplied:

-   Rise time: 2 ms,-   Pulse width: 40 ms,-   Fall time: 80 ms,-   Period: P=124 ms, including a 2 ms period in which the signal    presents its initial value,-   Minimum value=0 V,-   Maximum value=15 V.

The operation of the diagnostic device is illustrated in FIGS. 4 to 6.These figures show the signal V2 at the amplifier output, the voltageVdiag prevailing at a terminal of the coil 2 and the voltage Vmuxmeasured by the measurement logic circuit.

In FIG. 4, it is assumed that the antenna is correctly connected to itsdriver, and that there is therefore no short circuit. The numericalinformation on the x and y axes is given purely by way of indication. Onthe x-axis, the values correspond to milliseconds, whereas on they-axis, they represent Volts.

When the antenna is correctly connected, the coil 2 short circuits thecapacitor C2. The resistor R1 is linked to ground. Because of this, theground level rises to the point at which Vdiag is measured. During atransient state, the capacitors C1 and C3 are charged through R1 andthen C3 is discharged.

This is evident from FIG. 4 in which the voltage Vdiag corresponds tothe capacitor C3 charge. The voltage Vmux corresponds to the voltageVdiag, but with peak clipping due to the presence of the protectiondiodes of the multiplexer input stage.

The diagnostic procedure according to the present invention proposescarrying out a first measurement at a time t1 at which the voltage Vmuxis at its maximum or presents a value close to this maximum value. Thisfirst measurement is therefore performed during the transient stateduring which the capacitors C1 and C3 are charged and then discharged.This measurement must be performed during the period referenced P1 inFIG. 4. The procedure according to the invention then proposes carryingout a second measurement at a time t2 corresponding to a steady statewhen the capacitors are discharged. In the case of FIG. 4, the voltageVmux is then close to zero.

FIG. 5 represents the Vdiag, Vmux curves relative to the voltage V2 inthe case where the antenna is not connected, which then corresponds tothe case where the coil 2 in FIGS. 1 and 2 would quite simply beremoved. In this case, the three capacitors are mounted in series. Thesecapacitors are charged during the rise time of the signal V2 and thendischarged via the resistor R1. It will be noted here that the voltageVmux remains maximum throughout the pulse width. Here, also, twomeasurements are carried out: a first during the transient state duringwhich the capacitors are charged followed by a second measurement in thesteady state in the pulse period. The period P2 symbolized in FIG. 5corresponds to the period during which the second measurement is carriedout.

Finally, FIG. 6 represents the case where a terminal of the antenna isdirectly linked by a short circuit to ground as symbolically representedin FIG. 1. In this case, the voltages Vdiag and Vmux remain zero. Forthe two measurements carried out, a voltage Vmux of zero is thereforeobtained.

As can be seen, by providing for two measurements, one during thetransient state and one later, the antenna connection fault can, whereappropriate, be diagnosed. In practice, when the antenna is correctlyconnected, the voltage measured in the first measurement presents a highlevel and the voltage measured in the second measurement presents a lowlevel. When the antenna is disconnected, the first measurement gives ahigh result as in the second measurement. Finally, when a short circuitto ground is present, both measurements carried out give a low level.The case of a short circuit to positive can also be considered. In thiscase, the two measurements carried out give a high level, which istherefore equivalent to diagnosing an absent antenna.

To obtain a correct diagnosis, it is obviously essential to choosecarefully the times at which the measurements are carried out. In thepresent case, t1=5 ms and t2=15 ms are proposed as examples. Thesevalues naturally depend on the shape of the signal V2. The firstmeasurement is carried out at the end of the rise time or right at thestart of the pulse duration. In all cases, it will be noted that at thismoment the state is transient. The second measurement is carried outwhen the state is steady. This measurement can, for example, be carriedout toward the middle of the pulse duration during which V2 is at itsmaximum. It is essential for the second measurement to be carried outafter the transient state, and this applies in all cases (antennaproperly connected, antenna disconnected, short circuit to ground or topositive).

The diagnostic device described above used in conjunction with theprocedure described can therefore be used to reliably detect anincorrectly installed antenna before using it. The measurement iscarried out, for example, when the circuit is first powered up. Thediagnosis can be obtained quickly and inexpensively.

The present invention is not limited to the embodiments described aboveby way of nonlimiting examples. It also relates to all variants withinthe scope of those skilled in the art, within the context of thefollowing claims.

1. A diagnostic procedure relating to the connection of an antennaincluding a coil (2) linked on the one hand to a reference potential andon the other hand to an output of an amplifier (4), a first capacitor(C2) being mounted in parallel with the coil (2) and a second capacitor(C3) being inserted between a terminal of the coil (2) and the referencepotential, characterized in that it comprises the following steps: a)transmission of a signal by the amplifier (4), b) first measurement ofthe voltage at a terminal of the antenna during the transient stateprovoked by the transmission of the signal, and c) second measurement ofthe voltage at the same terminal of the antenna in the steady state. 2.The diagnostic procedure as claimed in claim 1, characterized in that itis carried out on powering up the antenna.
 3. The diagnostic procedureas claimed in claim 1, characterized in that the signal sent by theamplifier (4) is not modulated.
 4. The diagnostic procedure as claimedin claim 3, characterized in that the signal at the amplifier (4) outputpresents a rise time, followed by a pulse duration and finally a falltime.
 5. The diagnostic procedure as claimed in claim 4, characterizedin that the first measurement is carried out during the rise time. 6.The diagnostic procedure as claimed in claim 4, characterized in thatthe second measurement is carried out during the pulse duration.
 7. Thediagnostic procedure as claimed in claim 1, characterized in that theantenna is connected to an input stage of a multiplexer and in that aresistor (R2) positioned between the antenna and the input stage of themultiplexer limits the current in the multiplexer input stage.
 8. Thediagnostic procedure of claim 5, wherein the first measurement iscarried out during a second half of the signal rise.
 9. The diagnosticprocedure as claimed in claim 2, characterized in that the signal sentby the amplifier (4) is not modulated.
 10. The diagnostic procedure asclaimed in claim 5, characterized in that the second measurement iscarried out during the pulse duration.
 11. The diagnostic procedure asclaimed in claim 2, characterized in that the antenna is connected to aninput stage of a multiplexer and in that a resistor (R2) positionedbetween the antenna and the input stage of the multiplexer limits thecurrent in the multiplexer input stage.
 12. The diagnostic procedure asclaimed in claim 3, characterized in that the antenna is connected to aninput stage of a multiplexer and in that a resistor (R2) positionedbetween the antenna and the input stage of the multiplexer limits thecurrent in the multiplexer input stage.
 13. The diagnostic procedure asclaimed in claim 4, characterized in that the antenna is connected to aninput stage of a multiplexer and in that a resistor (R2) positionedbetween the antenna and the input stage of the multiplexer limits thecurrent in the multiplexer input stage.
 14. The diagnostic procedure asclaimed in claim 5, characterized in that the antenna is connected to aninput stage of a multiplexer and in that a resistor (R2) positionedbetween the antenna and the input stage of the multiplexer limits thecurrent in the multiplexer input stage.
 15. The diagnostic procedure asclaimed in claim 6, characterized in that the antenna is connected to aninput stage of a multiplexer and in that a resistor (R2) positionedbetween the antenna and the input stage of the multiplexer limits thecurrent in the multiplexer input stage.